Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828
Vol. 47 No. 4 December 2016
Performance and Analyses of Thick Soft Clay Deposit Improved by PVD with Surcharge Preloading and Vacuum Consolidation – A Case Study at CMIT
By P.V. Long, L.V. Nguyen and A.S. Balasubramaniam
ABSTRACT: Ground improvement using PVD was applied successfully for increasing foundation stability and controlling residual settlements of the container yard constructed on 35m thick soft clay deposit at CMIT, Vietnam. The treated area is about 40ha including vacuum consolidation combined with 6.3m embankment surcharge for a strip of 57 m along the river bank (VCA) and conventional surcharge preloading using 9.1m sand fill embankment for the remaining area. The monitored data indicated that PVD thickness of 3 mm arranged in spacing of 0.9 m to 1.2 m can be used successfully for improvement of thick soft clay deposit in both methods of embankment preloading with and without vacuum pumping. Performance of reduced embankment combined with vacuum pumping is very much better than that of conventional embankment preloading in terms of shortening construction time, reducing lateral displacement, increasing stability, and minimizing residual settlement. Back calculated ch value is dependent on the assumptions of smear effects including smear zone ratio, d/dm and permeability ratio, Rs=kh/ks. For ds/dm= 2 as commonly used, the back-calculated ch value is directly proportional to Rs and the value of Rs in vacuum consolidation seems significantly smaller than that in embankment preloading. Using the back-calculated results of compressibility and flow parameters, the time-settlements re-calculated by 1-D method are in very good comparison with measured data for both conventional preloading and vacuum consolidation considering the vacuum pressure as an induced vertical stress distributed uniformly in the PVD zone. Analyses of factor of safety from observed pore pressures during embankment construction illustrated that the commonly used stability chart as given by Wakita & Matsuo (1994) is too conservative for PVD improved soft ground. Secondary compression behavior of thick soft ground improved by PVD including back calculation for coefficient of secondary compression and estimation of long term residual settlement have also been provided.
KEYWORDS: Settlement, Soft clay, Ground improvement, PVD, Vacuum consolidation
